Effective control of organic-metal interfaces is critical for achieving high-performance polymer solar cells (PSCs). Ideally, the work-function (Φ) of the cathode and anode should be aligned with the energy of the photo-excited quasi-Fermi levels (EF) of organic semiconductors to create Ohmic contact for maxing achievable open-circuit voltage (Voc) and minimized energy barrier for charge-extraction. Although low Φ metal such as Ca (Φ=2.9 eV) has been proved to form good contact with bulk heterojunction (BHJ) layer as cathode, its vulnerability to environmental conditions undermines its use for practical applications. More stable metals like Al (Φ=4.28 eV) and Ag (Φ=4.57 eV) have been used as cathode, but their relatively high Φ often cause energy mismatch between BHJ blends and themselves, which results in lower Voc and device performance.
To alleviate this problem, proper interfacial engineering by inserting a thin layer between cathode and active layer has been vigorously explored. For example, inorganic materials such as LiF and Cs2CO3 and metal oxides (TiOx, ZnOx), and organic materials such as insulating poly(ethylene oxide) (PEO) and conjugated polyelectrolyte (CPE) have also been proved to be effective in improving Al cathode based device performance. In a recent study, 8.37% of PCE was reported by inserting polyfluorene derivative (PFN) between the high performance PTB7:PC71BM BHJ and Ca/Al. In addition, self-assembled fullerenes (e.g., PCBM capped PEG and fluorocarbon modified PCBM (F-PCBM)) have also been reported to increase P3HT:PCBM based device performance.
Despite that interface engineering has been performed for conventional PSCs, the performances obtained from Ag-based devices were usually lower than those using Ca/Al and Al cathode. This significantly limits the utilization of stable and reflective Ag as cathode for improving performance and stability of devices, though it is well-known Ag anode can be advantageous in inverted PSCs to facilitate the printing process.
On the other hand, fullerene-based materials not only can match well with the energy level of the lowest unoccupied molecular orbital (LUMO) of commonly used acceptor (e.g., PCBM), but also possess sufficiently deep highest occupied molecular orbital (HOMO) energy level, which make them as energetically ideal candidates for electron transport layer (ETL) to facilitate electron-selecting and hole-blocking in PSCs.
Despite the advances in the development of materials to enhance solar cell performance, a need exists to provide effective interfacial materials that are capable of adjusting the Φ of cathode to improve the contact with the BHJ layer, possess reasonable electron mobility to minimize electrical resistance across the interfacial layer, and have sufficient orthogonal solvent-processibility and film forming properties to avoid eroding into the BHJ layer. The present invention seeks to fulfill this need and provides further related advantages.